The present invention relates generally to alignment metrology and resolution measurement systems for imaging arrays within a housing, and in particular, to the alignment of an area CCD or CMOS optical imaging array within a camera housing.
Digital camera systems, which incorporate a variety of optical imaging array detector configurations, have become a common image capture technology. Imaging array technologies include for example, charged-coupled device (CCD) or metal-oxide semiconductor (CMOS) detectors. The imaging array is a geometrically regular array of typically millions of light-sensitive regions that capture and store image information in the form of localized electrical charge that varies with incident light intensity. Each of the light-sensitive arrays is called a pixel.
For certain high performance applications, digital camera systems require accurate positioning, or alignment, of the optical imaging array relative to an external reference coordinate system, such as the housing that it sits within. For example, in most surveillance platforms, several imaging systems of different types (for example, infra-red (IR), color, low light, short wave infra-red (SWIR)) all look at the same target either through common optics or through a multiple port arrangement. It is critical from a targeting point of view that these several imaging systems are all aligned; that is, bore-sighted. A SWIR laser may be used to designate a target, but this laser may only be visible to the SWIR sensor. However, the SWIR sensor is limited in what it can see, and the other imaging systems serve to distinguish or determine other factors about the target. The image data gathered by the individual imaging systems are then fused together to provide a composite image or view of the target and its surroundings. Thus, in this case, bore-sighting or alignment of the various imaging systems, including the digital lowlight CCD camera, is critical because misalignment may give rise to artifacts which can cause false alarms.
Lowlight imaging systems typically have low F-number optical paths. This means that the depth of focus can be very small. Therefore, any error along the optical axis (typically denoted as the Z-axis) or tilt of the imaging plane can cause focus problems, either globally across the entire image, or in certain regions of the image. This is another reason that accurate alignment of the imaging array with an external coordinate system is needed.
Knowing the relative position of the imaging array with respect to the external coordinate system provides several additional benefits. First, for those applications whose image processing includes fusion of data from more than one imaging systems, having systems of known alignment relative to an external coordinate system can allow for faster overall imaging processing. In particular, the image data sets from misaligned cameras will need to be individually aligned or manipulated prior to fusion or combination of the separate sets into a composite image. This data alignment step is time consuming and often causes delays in the data pipeline. A second advantage of having a well-aligned imaging array with respect to its housing or other external coordinate system is the maintenance and ease-of-use and cost factor: a camera assembly whose imaging array is well aligned to an external reference coordinate system such as its housing does not need external adjustment mechanisms nor does it need special procedures or specially skilled or trained personnel to align in situ.
Therefore, a need continues to exist for an apparatus and method of aligning the imaging array in all six degrees of freedom (x, y, and z directions and rotationally about these three axes) relative to an external reference coordinate system such as a reference plane and/or lines on the camera housing. The three axes of rotation may or may not be orthogonal to each other. FIG. 1 illustrates the relationship between the imaging array (imager) and the housing or external reference coordinate system.
Certain applications for high performance digital cameras require operation in harsh environments such as very cold climates (outer space, high altitudes) or very warm climates (deserts). This means that the cameras are expected to remain in calibration over these temperature ranges. Hence, the alignment of an imaging array must provide accurate measurements over a wide temperature range of operation. As with most metrology tooling and methods, the accuracy of an apparatus and method for aligning the imaging array must be demonstrated.
Finally, since high performance digital cameras may be produced in high volumes, the alignment metrology apparatus and method should be relatively quick and straightforward to apply. Assuming the apparatus is calibrated, a single setup and measurement per unit under test that provide errors in all six degrees of freedom are desirable. In particular, more ideal would be the ability to extract all the information necessary to calculate the positional errors in all six degrees of freedom from a single image.